The present invention relates to a display device in which images and characters are displayed by light emitting elements constituted by cathode-ray tubes.
In a conventional large scale color image display device such as an electric display board for a stadium or an advertising display which may be mounted above the roof or on a side of a building, a number of colored incandescent light bulbs are arranged in a preferred pattern and are selectively turned on and off to display a desired image. Such a large scale color display device suffers from several difficulties.
One of the difficulties is that it is difficult to reproduce certain colors satisfactory. This is due to the fact that the colors provided by a light bulb are determined by the spectrum of the red hot filament of the light bulb. That is, the color of the light emitted by the light bulb filament is red or yellow-orange. In order to obtain the three primary colors, red, green and blue, which are necessary to reproduce a color image, color filters are employed to obtain the three primary color lights. It is considerably difficult to obtain green and blue lights because the light emitted from the light bulb filament contains very little green light component and hardly any blue light component, thereby resulting in reducing of the electric power efficiency.
In the system of selectively turning on and off light bulbs described above, in order to modulate the brightness of each picture element it is necessary to interrupt the application of the constant amplitude current to the filament or to vary the current applied to the filament. If the brightness is attempted to be modulated by controlling the time width in the on-off control of the filament, to increase and decrease the average current, the result is to change the temperature of the filament and hence spectrum of the output lights because of the spectrum of the output light depends on the temperature of the filament. Accordingly, the emission spectrum varies depending on the brightness. That is, the emission spectrum at high brightness is different from that at low brightness (less blue component) which makes it difficult to reproduce an image with the correct hue. Light bulbs used in such applications have a power rating of the order of 12 W. Typically, more than several ten-thousands of light bulbs are used to form a single display device. Accordingly, the conventional display device involves many problems to be solved such as power consumption, heat generated and service life.